ENVIRONMENTAL EMBRITTLEMENT OF IRON ALUMINIDES

The effects of gaseous and liquid environments on the fracture behavior of Fe3Al and FeAl are reviewed. The influence of variables such as composition, microstructure, loading rate and temperature are described. It is shown that the iron aluminides, when tested in dry environments (oxygen, dry air, vacuum) are inherently ductile, but that the presence of very small levels of moisture is sufficient to cause severe embrittlement. Hydrogen, whether released from water vapor or directly in contact with the iron aluminides, is the agent of embrittlement. A detailed discussion of the mechanism of embrittlement is included. Alleviation of embrittlement by such techniques as alloying, surface protection and control of microstructure is outlined. Finally, the environmental embrittlement of iron aluminides is contrasted to that of other aluminides; the major difference is shown to be the propensity for cleavage fractures in FeAl and Fe3Al, compared to intergranular fracture in L1(2) polycrystals.